International patent specification WO 90/09168 discloses a controlled release device for delivering a substance to a patient at a chosen time following administration. The device comprises a water-swellable male plug engaged within the female body. A pharmaceutically active material is contained within the device. When the device is exposed to water, the male hydrogel plug swells and eventually disengages itself from the female body, thereby allowing the pharmaceutically active material contained within the device to be released. It has been found that the time taken to disengage the hydrogel plug (e.g. 0.5 to 12 hours) is predictable and reproducible, so that the device may be used to release pharmaceutically active materials within the body of a patient after a predetermined time interval. This may be useful in the treatment of medical conditions where it is desirable to administer a pharmaceutically active material to the patient sometime through the night while the patient is asleep, so as to provide a desired level of the drug in the patient in accordance with his needs during the night or when he awakes. It may also be useful to allow administration of materials at a predetermined point as the device passes through the gastro-intestinal tract, for example in the colon.
Patent specification WO92/13521 (Alza Corporation) describes fluid-imbibing dispensing devices for delayed delivery of an active agent, which include an expansion means which absorbs fluid from a surrounding environment. The dispensing device comprises a housing having first and second wall sections telescopically engaged with each other, particularly a capsule having a hollow cap and a hollow body; either the cap or the body is in the form of a male section fitted inside the open end of the other female section. The expansion means is contained within the device and expands as it absorbs fluid, forcing apart the two sections of the device. The expansion means may be a swellable polymer or an osmotic formulation which swells as it absorbs fluid. In order to allow fluid to come into contact with the expansion means contained within the device, one of the wall sections adjacent to the expansion means is fluid-permeable. After the sections are disengaged, fluid enters the device and comes into contact with the active agent contained within the device, thereby dispensing the active agent into the fluid.
In order for the device to function correctly, the active material contained within the female body should be released promptly after the male member has become disengaged. This is dependent on the active material dissolving rapidly in the aqueous medium whilst the active agent is still within the female body, or depends on the material being quickly flushed out of the hollow female body. This in turn requires an active agent which has good solubility in water and/or an adequate supply of aqueous medium. However, there may be instances where the supply of aqueous medium in the patient's body is limited. For example, in the colon insufficient aqueous medium may be present for the device to operate as desired.
U.S. Pat. No. 4,957,494 discloses a dispenser for use in a fluid environment which is intended to release pulses of active agent over a long time period. The construction comprises a housing containing a plurality of layers of active material which are driven towards an outlet of the housing by a fluid actuated driving means. The driving means may include a fluid-swellable polymer, and the portion of the housing next to the driving means has a controlled permeability to fluid i.e. water, such that the rate of swelling of the polymer is controlled by the rate of ingress of water. However, the arrangement is complex and is intended to release the active agent very slowly over an extended period (e.g. at least 21-30 days) rather than to provide a very rapid release of active material.
It is an object of the present invention to mitigate these problems, by providing a system which rapidly positively expels the contents from the body.